1. Field of the Invention
The present invention relates to a satellite transmission capturing method for use with GPS (global positioning system) receivers.
2. Description of the Prior Art
The typical prior art GPS receiver inversely diffuses satellite transmissions using PN code format, the transmissions coming from GPS satellites and being spectrum-spread. After the inverse diffusion, the GPS receiver demodulates the received signal which was phase shift keyed (PSK) in order to obtain desired navigation data. The PSK signal is demodulated as follows: The in-phase and orthogonal components of a reference carrier are multiplied by the PSK signal. The resulting products are each fed to a low pass filter to acquire an in-phase channel signal I(t) and an orthogonal channel signal Q(t), the former being the original base band signal. This demodulation method is called synchronous detection.
As described, to demodulate a PSK signal requires the use of a reference carrier synchronized in phase with the receiving frequency of a desired satellite transmission. However, because satellite transmissions are sent out after balanced modulation, they do not contain a carrier. For this reason, the GPS receiver generally uses a PLL (phase locked loop) arrangement to reproduce a reference carrier synchronized in phase with the receiving frequency of the target satellite transmission.
Transmissions (L.sub.1 wave) are sent from GPS satellites at 1575.42 MHz. Because GPS satellites are not geo-stationary but orbiting satellites and because the GPS receiver is mounted on a moving body such as a vehicle for on-the-move reception, the Doppler effect is experienced with satellite transmissions. That is, the actual frequency of satellite transmissions upon receipt on the ground varies in an unpredictable manner around the transmitting frequency of 1575.42 MHz. Thus in receiving and demodulating satellite transmissions with the GPS receiver on the ground, it is necessary to search the Doppler-shifted transmissions for the actual frequency and to have a demodulator reproduce a reference carrier synchronized in phase with that actual frequency.
FIG. 7 illustrates the operating principle of the typical prior art satellite transmission capturing method for use with GPS receivers. In FIG. 7, the horizontal axis stands for the frequency of the reproduced carrier of the demodulator, and the vertical axis denotes a demodulation intensity I.sup.2 +Q.sup.2 of the demodulator. That is, the demodulation intensity, which indicates the status of satellite transmission reception, is represented by the sum of a demodulated in-phase channel signal I(t) squared and a demodulated orthogonal channel signal Q(t) squared. f.sub.c represents the receiving frequency of the actual Doppler-shifted satellite transmissions. A demodulation intensity curve M indicating the output characteristic of the demodulator forms a horizontally symmetrical, single-peak characteristic curve around the center of the receiving frequency f.sub.c, as illustrated.
As shown in FIG. 7, when the frequency "f" of the reproduced carrier of the demodulator coincides with the receiving frequency f.sub.c of the satellite transmission, the demodulation intensity I.sup.2 +Q.sup.2 is maximized. The prior art GPS receiver searches the satellite transmissions for the receiving frequency f.sub.c as follows: Within the range of Doppler-shifted frequency variations, the reproduced carrier frequency of the demodulator is switched one step at a time, i.e., in increments of a PLL capture range .DELTA.f.sub.n starting from a search start frequency f.sub.s. When the receiving frequency f.sub.c of the satellite transmission falls within a given capture range .DELTA.f.sub.n, the PLL function is synchronized in phase and locked so as to obtain the reference carrier for demodulation, the carrier being synchronized in phase with the receiving frequency f.sub.c of the satellite transmission.
One disadvantage of the above-described prior art satellite transmission capturing method is that it takes time to capture the satellite transmission because the transmissions are searched even as the frequency "f" of the reproduced carrier of the demodulator is changed in increments of the PLL capture range .DELTA.f.sub.n.